This invention relates to catalytic cracking of hydrocarbons. It more particularly refers to improvements in the endothermic catalytic cracking of petroleum fractions and alternating exothermic catalyst regeneration.
Endothermic catalytic cracking of hydrocarbons, particularly petroleum fractions, to lower molecular weight desirable products is well known. This process is practiced industrially in a cycling mode wherein hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst without added hydrogen at rather low pressures of up to about 50 psig and temperatures sufficient to support the desired cracking. As the hydrocarbon feed is cracked to lower molecular weight, more valuable and desirable products, "coke" is deposited on the catalyst particles. The coked catalyst is disengaged from the hydrocarbon products, which are then resolved and separated into appropriate components. The coked catalyst particles, now cooled from the endothermic cracking and disengaged from the hydrocarbon products, are then contacted with an oxygen containing gas whereupon coke is burned off the particles to regenerate their catalytic activity. During regeneration, the catalyst particles absorb the major portion of the heat generated by the combustion of coke, i.e. they are "reflexively" heated, with consequent increase of catalyst temperature. The heated, regenerated catalyst particles are then contacted with additional hydrocarbon feed and the cycle repeats itself.
A flue gas comprising carbon oxides is produced during regeneration. In conventional operation this flue gas contains substantial quantities of carbon monoxide. The carbon monoxide is either vented to the atmosphere with the rest of the flue gas or is in some way burned to carbon dioxide, in an incinerator or a CO boiler or the like.
It has recently become desirable to decrease the content of carbon monoxide in the regenerator flue gas for at least two reasons. In the first place, CO combustion is extremely exothermic and in view of the increasing cost of energy, burning CO in the regenerator increases the heat efficiency of the reflexive endothermic catalytic cracking system. In the second place, since carbon monoxide is an air pollutant, more and more stringent controls are being placed upon its venting into the environment. It is therefore clearly desirable to provide means for burning carbon monoxide within a reflexive hydrocarbon catalytic cracking system. This has been attempted in the past and is being attempted at present by means of increasing the temperature and air input to the regenerator so as to support thermal combustion of carbon monoxide in the regenerator. This technique has been difficult to commercialize and to operate successfully in a smooth, steady state manner.
In the past attempts have been made, in fact it has sometimes been commercial practice, to employ special catalysts for this process which contain a cracking component and a component for catalyzing the oxidation of carbon monoxide. The CO oxidation components used in the past have been metals of the transition element group and/or of the iron group. In particular, manganese, cobalt and especially chromium have been used for this purpose.
Two major variants for endothermically cracking hydrocarbons are fluid catalytic cracking (FCC) and moving bed catalytic cracking. In both of these processes as commercially practiced, the feed hydrocarbon and the catalyst are passed through a "reactor"; are disengaged; the catalyst is regenerated with cocurrent and/or countercurrent air; and the regenerated reflexively heated catalyst recontacted with more feed to start the cycle again. These two processes differ substantially in the size of the catalyst particles utilized in each and also in the engineering of materials contact and transfer which is at least partially a function of the catalyst size.
In fluid catalytic cracking (FCC), the catalyst is a fine powder of about 10 to 200 microns, preferably about 70 micron, size. This fine powder is generally propelled upwardly through a riser reaction zone suspended in and thoroughly mixed with hydrocarbon feed. The coked catalyst particles are separated from the cracked hydrocarbon products, and after purging are transferred into the regenerator where coke is burned to reactivate the catalyst. Regenerated catalyst generally flows downward from the regenerator to the base of the riser.
One typical example of industrially practiced moving bed hydrocarbon catalytic cracking is known as thermofor catalytic cracking (TCC). In this process the catalyst is in the shape of beads or pellets having an average particle size of about one-sixty-fourth to one-fourth inch, preferably about one-eighth inch. Active, hot catalyst beads progress downwardly cocurrent with a hydrocarbon charge stock through a cracking reaction zone. In this zone hydrocarbon feed is endothermically cracked to lower molecular weight hydrocarbons while coke is deposited on the catalyst. At the lower end of the reaction zone the hydrocarbon products are separated from the coked catalyst, and recovered. The coked catalyst is then passed downwardly to a regeneration zone, into which air is fed such that part of the air passes upwardly countercurrent to the coked catalyst and part of the air passes downwardly cocurrent with partially regenerated catalyst. Two flue gases comprising carbon oxides are produced. Regenerated catalyst is disengaged from the flue gas and is then lifted, pneumatically or mechanically, back up to the top of the reaction zone.
The catalysts used in endothermic catalytic nonhydrogenative cracking are to be distinguished from catalysts used in exothermic catalytic hydrocracking. Operating conditions also to be distinguished. While the catalytic cracking processes to which this invention is directed operate at low pressures near atmospheric and in the absence of added hydrogen, hydrocracking is operated with added hydrogen at high pressures of up to about 1000 to 3000 psig. Further, non-hydrogenative catalytic cracking is a reflexive process with catalyst cycling between cracking and regeneration (coke burn off) over a very short period of time, seconds or minutes. In hydrocracking, on the other hand, the catalyst remains in cracking service for an extended period of time, months, between regeneration (coke burn off). Another important difference is in the product. Nonhydrogenative catalytic cracking produces a highly unsaturated product with substantial quantities of olefins and aromatics, and a high octane gasoline fraction. Hydrocracking, in contrast produces an essentially olefin-free product with a relatively low octane gasoline.
This invention is not directed to hydrocracking nor is it within the scope of this invention to use hydrocracking catalysts in the process hereof. Hydrocracking catalysts have an acidic cracking component, which may be a crystalline aluminosilicate zeolite, amorphous silica alumina, clays or the like, and a very strong hydrogenation/dehydrogenation component. Strong hydrogenation/dehydrogenation components are illustrated by metals such as molybdenum, chromium and vanadium, and group VIII metals such as cobalt, nickel and palladium. These are used in relatively large proportion, certainly large enough to support heavy hydrogenation of the charge stock under the conditions of hydrocracking. To the contrary, strong hydrogenation/dehydrogenation metals are neither required nor desired as components of non-hydrogenative catalytic cracking. In fact, it is usual for some metals, such as nickel and vanadium, to deposit out on the catalyst from the charge stock during non-hydrogenative cracking. These are considered to be catalyst poisons in this process and therefore to be avoided or at least minimized. Their detrimental effect in nonhydrogenative catalytic cracking is to increase the coke and light gas, including hydrogen, produced in the cracking reaction and therefore to reduce the yield of desired liquid products, particularly gasoline.